The invention relates to a method for the conveyance of a fluid by using centrifugal pumps, with machines and/or appliances which influence the pressure and/or temperature of the fluid being arranged upstream of a centrifugal pump. The invention relates, furthermore, to a method for the sequestration of carbon dioxide, the carbon dioxide being brought to a pressure and/or temperature suitable for deposit in an intended storage location and being conveyed into the storage location.
In the combustion of fossil fuels in power stations, carbon dioxide is produced which is critically responsible for the greenhouse effect. The aim, therefore, is to reduce the emission of carbon dioxide into the atmosphere. An effective measure is the sequestration of carbon dioxide. In this case, the carbon dioxide which has been produced in the power stations is separated and is delivered to a storage location. Appropriate storage locations include geological formations, such as petroleum deposits, natural gas deposits, saline groundwater aquifers or coal seams. Deep sea storage has also been investigated.
In conventional methods, the conveyance of gaseous carbon dioxide takes place by using compressors. Compression takes place in several stages, with various intermediate coolings of the compressed gas being necessary. Compression occurs from the gaseous state directly into the supercritical state. Both compression and cooling are highly energy-intensive.
Liquid carbon dioxide has also occasionally been conveyed by diaphragm pumps. If liquid carbon dioxide is pumped, then it is necessary to ensure that cavitation does not occur in the pump. The carbon dioxide should assume only states in which the vapor pressure is not reached or is undershot. Otherwise, the formation of vapor bubbles occurs, which implode in the event of overpressurizing the pump and lead to severe damage. The vapor pressure curve thus constitutes a boundary line for the conveyance of liquid carbon dioxide.
When liquid carbon dioxide is being conveyed, an unavoidable change to a supercritical state may occur in the pump. This is because of its relatively low critical temperature of only 31.0° C. and its relatively low critical pressure of only 73.8 bar. Furthermore, there are methods in which the carbon dioxide is in the supercritical state even when it enters the pump.
In principle, the conveyance of supercritical carbon dioxide by using centrifugal pumps is known. Jones et al., US 2005/0112003 (=WO 2005/052365) describes a single-stage canned motor pump which conveys the supercritical carbon dioxide in circulation. The fluid is conveyed by an impeller fastened on a shaft which is arranged in corrosion-resistant bearings. This is intended to prevent the formation of abrasive particles which may destroy the high-speed canned motor.
Forthuber, U.S. Pat. No. 6,224,355 (=WO 00/63529) describes a pump system for conveying liquid or supercritical carbon dioxide. The pump system comprises a multistage pump constructed in the manner of a submersible motor pump which is arranged in a pot housing. This arrangement relies on a closed conveying system in which very high pump inlet pressures prevail. Due to the boundary conditions mentioned, the carbon dioxide to be conveyed is present solely in the liquid phase. The system is used for enhanced oil recovery, EOR, in which carbon dioxide is injected into oilfields in order to increase the yield of conveyed oil. The system also serves for the sequestration of carbon dioxide.
In the conveyance of supercritical carbon dioxide by centrifugal pumps, serious problems often arise, since in the supercritical range, the carbon dioxide repeatedly assumes states which lead to discontinuous pumping behavior and sometimes also to damage to the centrifugal pump. In the event of a pressure rise in the centrifugal pump, pronounced changes in density of the fluid occur which cause this behavior.